A Decibel (abbreviated to dB) is one tenth of a Bel. A Bel (named in honour of Alexander Graham Bell - the man who invented the telephone) was the amount of signal lost in level over a one mile distance of telephone wire.

This amount of loss is too large to be used, hence the use of a tenth of it, the decibel.

The decibel (dB) represents a ratio, and must have a reference level to establish the 0 value. All other values therefore become greater or less than this level by a certain number of dB.

The dB is also logarithmic. I will not go into a discussion about logarithms here, but the important quality of logarithms is that they can express a wide range of values with a limited number of values. For example, a microphone may produce 0.00001 volts in response to a quiet sound, and 10 volts in response to a very loud one. This is a huge range of numbers, but in dB becomes a range of 0 to 130.

The ratios the dB expresses are related to power, either acoustic or electrical. There are a number of dB references in common usage in audio, and these are sumarised below: